1. Field of the Invention
The present invention relates to an optical transmission system and particularly to the optical transmission system to transmit an optical signal according to a WDM (Wavelength Division Multiplex) method.
2. Description of the Related Art
In recent years, as demands for communication using broadband service provided by the Internet increases, transmission distance is made longer and transmission capacity is made larger in an optical communication network, and research and development of such a WDM (Wavelength Division Multiplex) transmission method, in which light having a different wavelength is multiplexed and a plurality of signals is simultaneously transmitted by one piece of an optical fiber, as can achieve high speed transmission and large capacity exceeding 40 Gbps are progressing. On the other hand, since transmission speed of optical signals by optical fibers differs in every wavelength of light, chromatic dispersion causing a pulse waveform of light to be blunt occurs as transmission distance of light increases. In the WDM system to achieve large-capacity and long-distance optical transmission, pulse expansion caused by chromatic dispersion remarkably lowers a signal receiving level and causes a harmful effect on the system. To solve this problem, an inverse dispersion is added to the received optical signal, so that the chromatic dispersion be equivalently zero (or be cancelled). A conventional and typical method for this dispersion compensation is to provide a DCF (Dispersion Compensation Fiber) in each of repeater sections and to suppress variations in chromatic dispersion.
FIG. 12 shows a diagram conceptionally explaining the WDM system that makes dispersion compensation by using the DCF. Nodes 50 and 60 are connected to each other through an optical fiber f1. The node 50 includes a post-emplifier 51 and the node 60 includes a pre-amplifier 61 and a DCF 62. The post-amplifier is an optical signal transmission amplifier to amplify a WDM signal obtained after processing in a node. The pre-amplifier is an optical signal receiving amplifier to receive a WDM signal transmitted from a preceding-stage node.
The WDM signal processed in the node 50 is amplified by the post-amplifier 51 and is then transmitted to the optical fiber f1. The WDM signal flown through the optical fiber f1, when reaching the node 60, passes through the DCF 62 and, after being amplified by the pre-amplifier 61, is processed in the node 60.
Thus, the chromatic dispersion of the WDM signal that produced a chromatic dispersion value (+D) by being flown through the optical fiber f1 is cancelled by passing through the DCF 62 having a chromatic dispersion value (−D). However, in the dispersion compensation using the above DCF, since the repeater sections of the node are variable (in land optical communication network in particular), there are variations in the chromatic dispersion values occurring in each of the repeater sections, it is necessary that the DCF that can correspond to the chromatic dispersion value is placed in each of the repeater sections.
Moreover, the chromatic dispersion value, due to its dependency on temperatures, varies depending on changes in temperature between day and night and among seasons and, in the WDM system handling a transmission speed of 40 Gbps, it is difficult to make dispersion compensation with high accuracy only using such a fixed dispersion value as the DCF.
On the other hand, a variable dispersion compensator called a VIPA (Virtually Imaged Phasesd Array) has been developed recently. The VIPA is an optical device having a chromatic dispersion element (VIPA plate) obtained by coating both sides of a thin plate such as a glass plate with a reflection film and a reflection mirror, which enables dispersion compensation of all wavelength bands of a WDM signal.
FIG. 13 shows a diagram conceptionally explaining the WDM system to make dispersion compensation using the VIPA. The nodes 50 and 70 are connected to each other through the optical fiber f2. The node 70 includes a pre-amplifier 71 and a VIPA 72. In such a system, a WDM signal flown from the node 50 undergoes dispersion compensation in every wavelength by the VIPA 72. The VIPA 72 has a feature that an amount of dispersion compensation can be set optimally in a manner to follow secular changes such as temperature variations in an optical fiber.
Thus, unlike the DCF which makes fixed dispersion compensation, the VIPA can achieve, in a variable manner, collective dispersion compensation over all wavelengths of a WDM signal and, therefore, construction of a WDM system based on automatic dispersion compensation technology using the VIPA is greatly expected.
Dispersion compensation technology, using a VIPA, for chromatic dispersion occurring when signals propagate through an optical fiber is disclosed in, for example, Japanese Patent Application Laid-open No. 2000-511655 (Pages 29 to 32, FIG. 14). In a WDM system, it is necesary that a gain of a pre-amplifier is set in an initial stage of operations of the WDM system and, in this case, some input light having a light level required for the gain setting is fed to the pre-amplifier. In ordinary cases, a gain is set by inputting an ASE (Amplified Spontaneous Emission) light, which is output from a post-amplifier being placed in a preceding-stage node, having a light level being equivalent to one wave of a WDM signal. Here, the optical amplifier including the post-amplifier and pre-amplifier as described above is generally made up of an EDFA (Erbium-dopped Fiber Amplifier). The EDFA is an optical amplifier using an EDF [Eribium(Er3+)-dopped Fiber] as a medium for amplification in which an optical signal is made to travel by irradiating the EDF with pumping light and, by using simulated emission occurring at the time of the irradiation, a level of an optical signal is amplified.
In the optical amplifier such as an EDFA using simulated emission as operational principles of amplification, irrespective of existence of light input to the optical amplifier, a phenomenon of spontaneous emission occurs. The light having leaked from the optical amplifier by the phenomenon becomes noise light which is the ASE light.
Moreover, a light level of the ASE light can be adjusted by a user. Therefore, by adjusting a post-amplifier in advance so as to have an optical level being equivalent to one wave of a WDM signal, ASE light at a desired level required for booting the pre-amplifier is emitted spontaneously from the post-amplfier at the time of an initial operation of the WDM system.
FIG. 14 shows a diagram showing a light level of one wave of a WDM signal and an ASE light level. In FIG. 14, a light level is plotted as ordinate and a wavelength as abscissa. The ASE light 82 has a light level being equivalent to one wave of a WDM signal by a bandwidth of all wavelengths of the WDM signal (the ASE light 82 and WDM signal 81 of one wave are the same in areas). The pre-amplifier receives the ASE light 82 as described above to automatically set a gain.
In the WDM system using the DCF as shown in FIG. 12, even if the ASE light 82 passes through the DCF 62, no change occurs in the level of the ASE light 82 and it is possible to normally do the gain setting of the pre-amplifier 61. However, the WDM system using the variable dispersion compensator such a VIPA 72 as shown in FIG. 13 has a problem in that, when the ASE light 82 is input to the VIPA 72 being placed in the preceding-stage of the pre-amplifier 71, a light level of the ASE light 82 is lowered to become a level value being different from a light level being equivalent to one wave of the WDM signal and normal setting of a gain of the pre-amplifier 71 cannot be done.
FIG. 15 shows a diagram conceptually explaining a level of the ASE light obtained after having passed through the VIPA. A light level is plotted as ordinate and a wavelength as abscissa. When a waveform of the ASE light 82 input to the VIPA 72 and obtained after having output from the VIPA 72 is observed, the ASE light 83 having a comb-shaped waveform as shown in FIG. 15 is seen. The level of the ASE light 83 is lowered when compared with the light level of the ASE light 82 as shown in FIG. 15 (an area of the ASE light 83 is smaller by an area being cut). The conventional WDM system has a problem in that, when such ASE light 83 is input to a pre-amplifier which requires a light level being equivalent to one wave of a WDM signal, an accurate setting of a gain is impossible, causing quality of the WDM system to be degraded.
Moreover, as an example of a device which causes a level of the ASE light to change when the ASE light is made to pass through the device, the VIPA is shown in the above embodiment, however, such a device is not limited to the VIPA. When the ASE light is made to pass through a device whose filtering function has wavelength periodicity, a level of the ASE light changes as shown in FIG. 15. (a phenomenon in which wavelength periodicity appears when light having a wide band (white light) is incident, light is output in a transmission manner in the order of λ1, λ2, λ3 . . . is called “wavelength periodicity” and optical devices such as a VIPA, etalon, or the like provide a filtering function having the wavelength periodicity as described above).